Multilayered lead frame for a semiconductor light-emitting device

ABSTRACT

A lead frame ( 100 ) for a semiconductor device is formed by applying nickel plating ( 102 ), palladium plating ( 103 ), and gold flash plating ( 104 ) substantially entirely to lead frame body ( 101 ) such as copper thin plate in this order, and further applying silver plating ( 105 ) selectively to part of an inner part that is to be enclosed with a package of the semiconductor device. The lead frame ( 100 ) may also include a base of the package. The silver plating contributes to an excellent light reflectance and wire bonding efficiency of the inner part, whereas the gold flash plating contributes to an excellent resistance to corrosion and soldering efficiency of an outer part that is outside the package.

RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.10/542,419, filed Dec. 29, 2005 now U.S. Pat. No. 7,692,277, which is aU.S. National Phase under 35 U.S.C. §371 of International ApplicationNo. PCT/JP2004/000152, filed on Jan. 13, 2004, which in turn claims thebenefit of Japanese Application No. 2003-007988, filed on Jan. 16, 2003,the entire contents of each of which are hereby incorporated byreference.

TECHNICAL FIELD

The present invention relates to a lead frame for a semiconductordevice, and in particular relates to techniques of improving a lightreflectance of a lead frame.

BACKGROUND ART

Conventionally, gold-silver alloy flash plating is formed at anoutermost layer of a lead frame for a semiconductor device, to ensure ahigh resistance to corrosion. An example of such techniques is disclosedin Unexamined Japanese Patent Application Publication No. H11-008341.

FIG. 6 is a representation of a construction of a lead frame describedin the above document. In the drawing, a lead frame 900 is formed byapplying nickel plating 902, palladium plating 903, and gold-silveralloy flash plating 904 to a lead frame body 901 in this order.According to the document, the lead frame 900 having this constructionexhibited an excellent corrosion resistance in a salt-spray test.

However, though this construction certainly has a high corrosionresistance, it has only a low light reflectance. This being so,especially when a semiconductor light-emitting element such as an LED ismounted on the lead frame to form a semiconductor light-emitting device,the lead frame cannot sufficiently reflect rear light of thelight-emitting element, which impairs the luminous efficiency of theentire device.

DISCLOSURE OF THE INVENTION

The present invention was conceived in view of the problem describedabove, and aims to provide a lead frame for a semiconductor device withan improved light reflectance.

The stated object can be achieved by a lead frame for a semiconductordevice, including: a lead frame body; and a plurality of metal coatingswhich include a silver or silver-alloy coating and are applied to thelead frame body, wherein the silver or silver-alloy coating is anoutermost metal coating of a predetermined part of the lead frame, thepredetermined part being to be enclosed with a package of thesemiconductor device.

Here, the plurality of metal coatings may further include a gold orgold-alloy coating which is an outermost metal coating of a part of thelead frame other than the predetermined part.

Here, the silver or silver-alloy coating may have a thickness of 0.1 μmor more.

Here, a base that is a part of the package may be inserted to the leadframe with a portion of the predetermined part being enclosed with thebase.

According to these constructions, the silver or silver-alloy coatingdelivers an excellent light reflectance and wire bonding efficiency ofthe predetermined part which is enclosed with the package, and the goldor gold-alloy coating delivers an excellent resistance to corrosion andsoldering efficiency of the other part which is outside the package.

Thus, the lead frame exhibits a high light reflectance in thepredetermined part. Accordingly, when used in a semiconductorlight-emitting device, the lead frame efficiently reflects rear light ofa semiconductor light-emitting element, with it being possible toenhance the luminous efficiency of the entire device.

Here, the gold or gold-alloy coating may substantially entirely coverthe lead frame body, wherein the silver or silver-alloy coating isapplied over the gold or gold-alloy coating only in the predeterminedpart.

According to this construction, the silver or silver-alloy coatingpartially covers the lead frame body, whereas the gold or gold-alloycoating entirely covers the lead frame body. This simplifies themanufacturing process of the lead frame.

Here, the plurality of metal coatings may further include a nickel ornickel-alloy coating and a palladium or palladium-alloy coating, whereinthe nickel or nickel-alloy coating, the palladium or palladium-alloycoating, the gold or gold-alloy coating, and the silver or silver-alloycoating are applied in the stated order, with the nickel or nickel-alloycoating and the palladium or palladium-alloy coating substantiallyentirely covering the lead frame body.

According to this construction, the palladium or palladium-alloy coatinghas a high high-temperature stability, which makes the lead framesuitable for high-temperature leadless soldering.

Here, the plurality of metal coating may further include a gold orgold-alloy coating, wherein the silver or silver-alloy coating maysubstantially entirely covers the lead frame body, and the gold orgold-alloy coating is applied over the silver or silver-alloy coatingonly in the part of the lead frame other than the predetermined part(FIG. 7).

According to this construction, the gold or gold-alloy coating partiallycovers the lead frame body, whereas the silver or silver-alloy coatingentirely covers the lead frame body. This simplifies the manufacturingprocess of the lead frame.

Here, the plurality of metal coatings may further include a nickel ornickel-alloy coating and a palladium or palladium-alloy coating, whereinthe nickel or nickel-alloy coating, the palladium or palladium-alloycoating, the silver or silver-alloy coating, and the gold or gold-alloycoating are applied in the stated order, with the nickel or nickel-alloycoating and the palladium or palladium-alloy coating substantiallyentirely covering the lead frame body.

According to this construction, the palladium or palladium-alloy coatinghas a high high-temperature stability, which makes the lead framesuitable for high-temperature leadless soldering.

Here, the silver or silver-alloy coating may be applied except at leastone part of the portion enclosed with the base.

According to this construction, the adhesion between the lead frame anda resin that forms the package is strong in the area where the silver orsilver-alloy coating is absent in the predetermined part. This maintainsthe seal of the package and improves the resistance to corrosion in thepredetermined part.

Here, a semiconductor light-emitting element may be mounted on the leadframe to form a semiconductor light-emitting device.

According to this construction, the semiconductor light-emitting deviceis formed using the lead frame that has an excellent light reflectancein the predetermined part. This makes it possible to efficiently reflectrear light of the semiconductor light-emitting element, whichcontributes to a high luminous efficiency of the entire device.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a top view of a lead frame to which an embodiment of theinvention relates.

FIG. 2 is a representation of a plating construction of the lead frameshown in FIG. 1.

FIG. 3 is a perspective view of the lead frame after a package isformed.

FIG. 4 is a representation of a cross section of a semiconductorlight-emitting device that uses the lead frame.

FIG. 5 is a graph for comparing reflectances of lead frames.

FIG. 6 is a representation of a plating construction of a conventionallead frame.

FIG. 7 is a representation of a plating construction of the lead frameof the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention is described in detail below, byreferring to the drawings.

(Pattern of a Lead Frame)

FIG. 1 is a top view of a lead frame to which the embodiment of thepresent invention relates.

In the drawing, a lead frame 100 is formed by pressing or etching a leadframe body in the illustrated pattern and then plating the lead framebody as described later. For example, the lead frame body is a thinplate of an iron alloy or a copper alloy.

A package for enclosing part of the lead frame 100 and housing asemiconductor element such as a semiconductor light-emitting element isprovided in each of the areas indicated by the dashed line boxes. Thepackage includes a base and a cover. The base has a depression formounting the semiconductor light-emitting element. The cover seals thedepression on which the semiconductor light-emitting element is mounted.

In this specification, a part of the lead frame 100 which is inside eachof the areas indicated by the dashed line boxes, i.e. which is enclosedwith the package, is called an inner part, whereas a part of the leadframe 100 which is outside the package is called an outer part. Theinner part includes inner leads. The inner part may also include anelement mounting unit that is provided separately from or as part of theinner leads.

(Plating)

FIG. 2 is a representation of a plating construction of the lead frame100, taken along the lines A-A′ in FIG. 1.

The lead frame 100 is formed by applying nickel plating 102, palladiumplating 103, and gold flash plating 104 to a lead frame body 101 in thisorder, and further applying silver plating 105 to part of the innerpart.

For example, the nickel plating 102 has a thickness of 0.5 to 2.0 μm,the palladium plating 103 has a thickness of 0.005 to 0.07 μm, the goldflash plating 104 has a thickness of 0.003 to 0.01 μm, and the silverplating 105 has a thickness of 0.1 μm or more.

Here, these plating layers may instead be made of alloys. Which is tosay, the nickel plating 102 may be nickel-alloy plating, the palladiumplating 103 may be palladium-alloy plating, the gold flash plating 104may be gold-alloy flash plating, and the silver plating 105 may besilver-alloy plating.

After plating, the base of the package is inserted to the area indicatedby the dashed lines. The base is made of a white-colored orlight-colored insulating resin, such as polyphthalamide. The base has adepression as shown in the drawing, and releases light of thesemiconductor light-emitting element placed in the depression in anupward direction. After the semiconductor light-emitting element isplaced in the depression, the depression is filled with a sealingtransparent resin (e.g. an epoxy resin) to form the cover. The base andthe cover together constitute the package.

It should be noted here that the silver plating 105 only partiallycovers the inner part, i.e., the part of the inner part enclosed withthe base is partially not plated with silver.

(Semiconductor Device Using the Lead Frame 100)

FIG. 3 is a perspective view of the lead frame 100 after a base 200 isformed.

FIG. 4 is a representation of a cross section of a semiconductorlight-emitting device 700 which uses the lead frame 100.

The semiconductor light-emitting device 700 is formed by mounting asemiconductor light-emitting element 400 on part of the inner partexposed to the depression of the base 200, connecting the semiconductorlight-emitting element 400 with the inner part using bonding wires 500,and then enclosing a transparent resin 300 in the depression to form acover.

Light 601 of the semiconductor light-emitting element 400 is directlyreleased upward, whereas other light 602 of the semiconductorlight-emitting element 400 is reflected off the inner part and the base200 and then released.

The inventors of the present invention confirmed that such a constructedlead frame has the following excellent properties.

(Light Reflectance)

FIG. 5 is a graph for comparing light reflectances of lead frames. Alight reflectance of a lead frame of each of sample 1 (the silver-platedlead frame part of the present invention), sample 2 (a comparativeexample), and sample 3 (a conventional example) was measured using avisible-ultraviolet spectrophotometer, where a light reflectance ofbarium sulfate is 100.

Sample 1 has the construction described above.

Sample 2 is formed by applying copper strike plating and silver platingthat has a thickness of 3 μm to a copper alloy thin plate as a leadframe body, in this order. Sample 2 is used as a comparative examplethat can deliver a light reflectance similar to the present invention,by single-layer silver plating.

Sample 3 is a conventional lead frame. Sample 3 is formed by applyingnickel plating of 1.0 to 1.2 μm in thickness, palladium plating of 0.03μm in thickness, and gold flash plating of 0.008 μm in thickness to acopper alloy thin plate as a lead frame body, in this order.

As shown in the graph, the silver-plated lead frame part of the presentinvention has a light reflectance that is at least 25% higher than thatof the conventional example and a little higher than that of thecomparative example, for visible light of 400 to 700 nm in wavelength.

(Resin Adhesiveness)

Also, the inventors of the present invention measured a shear adhesivestrength of the contact surfaces of a resin which forms the base and apalladium-plated lead frame part and a shear adhesive strength of thecontact surfaces of the resin and a silver-plated lead frame part. As aresult, the inventors found that the shear adhesive strength of theresin and the palladium-plated lead frame part is greater than that ofthe resin and the silver-plated lead frame part. In view of this, silverplating is provided only partially in the inner part, that is, silverplating is provided in the inner part except a part of the area that isenclosed with the base.

As a result, the resin adheres to the palladium plating layer throughthe gold flash plating layer, in the portion of the inner part which isnot plated with silver. This produces a greater shear adhesive strengththan in the portion of the inner part which is plated with silver. Sincethe gold flash plating layer is extremely thin, it does notsignificantly affect the shear adhesive strength.

The inventors of the present invention conducted an experiment ofsoaking samples 1 and 2 in red ink under the same conditions. As aresult, red ink did not seep into sample 1 which is not plated withsilver in part of the area enclosed with the base, whilst red ink seepedinto sample 2 which is entirely plated with silver.

(Other Properties)

In addition, each of the plating layers of the lead frame has thefollowing excellent properties.

The silver plating not only achieves an excellent light reflectance asdescribed above, but also has excellent connectivity with thesemiconductor light-emitting element mounted thereon and a high wirebonding efficiency.

The gold flash plating has a high thermal stability, which enhances thesoldering efficiency of the outer part.

The palladium plating has a high chemical stability, and so exhibits anexcellent resistance to corrosion under high-temperature environments.

The nickel plating serves as base plating, and contributes to a highwire bonding efficiency, a high leadless soldering efficiency, a highcorrosion resistance, and a high adhesive strength with the resin thatforms the package.

As described above, the lead frame of the present invention has acharacteristic construction in which silver or silver-alloy plating isapplied as the outermost coating of the inner part, to achieve afavorable light reflectance.

In the above embodiment, this characteristic construction is realized bycoating the lead frame body substantially entirely with nickel plating,palladium plating, and gold flash plating in this order, and furthercoating the inner part of the lead frame with silver plating. However,the construction of the present invention can equally be realized bycoating the lead frame body substantially entirely with nickel plating,palladium plating, and silver plating, and further coating the outerpart of the lead frame with gold flash plating. Hence this lead frame isincluded in the present invention, too.

In the above embodiment, each of the metal coatings applied to the leadframe body is referred to as plating for the sake of simplicity.However, the metal coating method of the present invention is notlimited to plating. For example, the metal coatings may be formed usinga known metal coating method such as electroplating, chemical plating,evaporation, sputtering, or diffusion.

Here, to coat a desired area with a metal, a conventional method such asmasking or blasting is applicable.

Industrial Applicability

The present invention can be used for a semiconductor light-emittingdevice that requires a high luminous efficiency, such as an LED deviceemployed in lighting, indication, decoration, communication, and thelike.

1. A semiconductor light-emitting device comprising: a package thatincludes a cover made of a transparent resin and a base made of aninsulating resin whose refractive index is different from a refractiveindex of the cover; a lead including an inner part enclosed in thepackage and an outer part that is outside the package; and asemiconductor light-emitting element mounted on the inner part of thelead, wherein: an outermost coating of the inner part of the leadincludes a first area and a second area, the first area including asilver or silver-alloy and the second area including a gold orgold-alloy, the cover is in contact with the first area but not incontact with the second area, and an outermost coating of the outer partis a gold or gold-alloy coating, the lead includes a nickel ornickel-alloy coating and a palladium or palladium ally coating, thenickel or nickel-alloy coating the palladium or palladium-alloy coating,and the silver or silver-alloy coating substantially entire cover thelead, in the fist area the nickel or nickel-alloy coating, the palladiumor palladium alloy coating and the silver or silver-alloy are applied inthe stated order, and in the second area the nickel or nickel-alloycoating, the palladium or palladium-alloy coating, the silver orsilver-alloy coating and the gold or gold-alloy are applied in thestated order.